Why not remove kneewall batting and replace with foam board?

I've been trying to go over all the options of the dreaded bonus room with kneewall issus. I live in central NC, so relativlely mild winters (but it still has that room freezing) and intense summers. The storage areas to both sides of the bonus room, the ones with the kneewall are floored already with OSB with batting under them. The walls have backed batting that sticks out 2" from the 2x4 studs. I have already sealed everything, the outlets and the top and bottom plates, with foam. But I was wondering, why not just replace that fiberglass between each stud with 3 or 4 layers of blue board and then across all the studs horizontally, nail up foil faced polyso? Wouldn't that be the best insualtion possible? 3" of foam board caulked around all 4 sides and then sealed over that with reflective coated foam? Or maybe don't worry about sealing each space between the studs and just seal up the foil board facing sheet all around to create a sealed wall.

Initially, I was just going to leave things as is, and then loosley staple perforated radiant barrier over the entire attic facing wall so it wouldn't compress the existing insulation very much. Not sure how much foam board I would need yet and how expensive it would be, but maybe when I do I might have to give up on this idea anyway

Or what about just nailing up and sealing the foam foil board over what I have, compressing the insulation a little bit, but then having 1" of foam board with foil that I seal all around?

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Why not remove kneewall batting and replace with foam board?

Careful w/ the foil faced foam. It will be your second (or at least one, in the wrong place) vapor barrier and you may get moisture issues. If you want to use it, put it in against your sheet rock, foil in. Sure, take the fiberglass out and foam it, like you said. Great R value; expensive, but it is sometimes the best product for the situation. This is a fixed cost, one-time payment deal, and lots of work. Put is the best insulation and forget the pennies difference in price. Good insulation pays for itself. As for the radiant barrier, I'd read on buildscience.com, greenbuildingadvisor.com, etc. It may or may not be of any value to you, but I'd tend to think "not". If it is a vapor barrier (I suspect so) then be careful about that, too.